This invention relates to surface wave devices and more particularly to surface wave frequency selective devices including a single interdigital transducer disposed on a piezoelectric substrate for attenuating a particular frequency or range of frequencies.
The prior art discloses a number of surface wave filter designs such as may be used in the intermediate frequency channel of a signal receiver such as a television receiver. Such devices typically include an input interdigital transducer and one or two output interdigital transducers which together have a band pass characteristic. By appropriate selection of the numbers of fingers, widths of fingers, and spacing between fingers as well as other considerations the shape of the pass band can be controlled to provide a reasonable approximation to the desired intermediate frequency response of the signal receiver. In the case of television receivers, a provision must be made for attenuating the audio carrier to a predetermined level relative to the video carrier. In the NTSC system used in the United States this provision has taken the form of a "trap" response which sharply attenuates the audio carrier. While such filtering has provided generally satisfactory performance, the automatic frequency control (AFC) must hold the frequency of the radio frequency (RF) oscillator within a very narrow band of frequencies to provide satisfactory audio carrier trapping.
In systems used in various other countries such as the CCIR system used in Germany and similar systems used in other countries, a sound-shelf type of response is required. Known prior art surface wave intermediate frequency filters have been unable to meet this system requirement necessitating the use of such expedients as discrete circuits thereby requiring tuning and periodic retuning with attendant set-up and service problems.